4 Introduction Products made of sheet metals are commonPressworking or press forming is used for general sheet-forming operations, as they are performed on presses using a set of dies

5 IntroductionA sheet-metal part produced in presses is called a stampingLow-carbon steel has low cost and good strength and formability characteristicsManufacturing processes involving sheet metal are performed at room temperature

7 Sheet and Plate Metal ProductsSheet and plate metal parts for consumer and industrial products such asAutomobiles and trucksAirplanesRailway cars and locomotivesFarm and construction equipmentSmall and large appliancesOffice furnitureComputers and office equipment

10 Three Basic Types of Sheet Metal Processes1.Cutting (Shearing)Shearing to separate large sheetsBlanking to cut part perimeters out of sheet metalPunching to make holes in sheet metal2.BendingStraining sheet around a straight axis3.DrawingForming of sheet into convex or concave shapes

14 Shearing Processing parameters in shearing areThe shape of the punch and dieThe speed of punchingLubricationThe clearance, c, between the punch and the dieWhen clearance increases, the zone of deformation becomes larger and the sheared edge becomes rougherExtent of the deformation zone depends on the punch speedHeight, shape, and size of the burr affect forming operations

18 Shearing Clearance Clearance control determine quality of its shearededges which influence formability of the sheared partAppropriate clearance depends on:1. Type of material and temper2. Thickness and size of the blank3. Proximity to the edges of other sheared edgesWhen sheared edge is rough it can be subjected to aprocess called shaving

19 Shearing: Characteristics and Type of Shearing DiesPunch and Die ShapePunch force increases rapidly during shearingLocation of sheared regions can be controlled by beveling the punch and die surfaces

22 Blanking and PunchingBlanking - sheet metal cutting to separate piece (called a blank) from surrounding stockPunching - similar to blanking except cut piece is scrap, called a slug

23 Shearing: Shearing OperationsPunching is where the sheared slug is scrapBlanking is where the slug is the part to be used and the rest is scrapDie CuttingShearing operation consists of:Perforating: punching holes in a sheetParting: shearing sheet into piecesNotching: removing pieces from the edgesLancing: leaving a tab without removing any material

25 Shearing Operations: SlittingShearing operations are through a pair of circular blades, follow either a straight line, a circular path, or a curved path

26 Shearing Operations: NibblingA contour is cut by a series of overlapping slits or notchesSimple tools can be used to produce complex shape

27 Shearing: Tailor-welded BlanksLaser-beam butt welding involves two or more pieces of sheet metal with different shapes and thicknessesThe strips are welded to obtain a locally thicker sheet and then coiledThe welded assembly is then formed into a final shape.Resulting in:Reduction in scrapElimination of the need for subsequent spot weldingBetter control of dimensionsImproved productivity

28 Shearing: Tailor-welded BlanksEXAMPLE 16.2Tailor-welded Sheet Metal for Automotive ApplicationsProduction of an outer side panel of a car body is by laser butt welding and stamping

30 Characteristics and Type of Shearing DiesTransfer DiesSheet metal undergoes different operations arranged along a straight line or a circular pathTool and Die MaterialsTool and die materials for shearing are tool steels and carbidesLubrication is needed for reducing tool and die wear, and improving edge quality

31 Characteristics and Type of Shearing DiesProgressive Diesa different operation is performed at the same station with each stroke of a series of punches

32 Characteristics and Type of Shearing DiesFor high product production ratesThe part shown below is the small round piece that supports the plastic tip in spray cans

33 Characteristics and Type of Shearing DiesCompound DiesOperations on the same sheet may be performed in one stroke with a compound dieLimited to simple shapes due to:Process is slowComplex dies is more expensive

35 Sheet-metal Characteristics and FormabilityYield-point elongation having both upper and lower yield points.This behaviour results in Luder’s bandsTypically observed with mild-steel sheetsResults in depressions on the sheet surfaceCan be eliminated by temple rolling (but sheet must be formed within a certain time after rolling)

37 Formability Tests for Sheet MetalsSheet-metal formability is the ability of the sheet metal to undergo the desired shape change without failureSheet metals may undergo 2 basic modes of deformation: (1) stretching and (2) drawingCupping TestsIn the Erichsen test, the sheet specimen is clamped and round punch is forced into the sheet until a crack appearsThe punch depth is a measure of formability of the sheetEasy to perform, but does not simulate exact conditions of actual forming, and not reliable for complex parts.

38 Formability Tests for Sheet MetalsForming-limit DiagramsTo develop a forming-limit diagram, the major and minor engineering strains are obtainedMajor axis of the ellipse represents the major direction and magnitude of stretchingMajor strain is the engineering strain and is always positiveMinor strain can be positive or negativeCurves represent the boundaries between failure and safe zones

39 Formability Tests for Sheet MetalsForming-limit DiagramsForming-limit diagrams is to determine the formability of sheet metals

40 Forming-Limit Diagram (FLD)Blank stretched over a punch, deformation observed and measured in the region where failure has occurredThe curves represent the boundaries between failure and safe zonesDifferent mat'ls have different FLDs, and the higher the curve, the better is the formabilityCompressive minor strain is associated with a higher major strain than a tensile minor strain of the same magnitudeThe effect of sheet-metal thickness is to raise the curvesFriction, lubrication at punch/sheet-metal interface, and surface scratches, are important factors

41 Bending Sheets , Plates, and TubesBending is a common industrial forming operationBending imparts stiffness to the part by increasing its moment of inertiaOuter fibers are in tension, while the inner in compressionPoisson effect cause the width to be smaller in the outer region and larger in the inner region

43 Bending Sheets, Plates, and TubesMinimum Bend RadiusIncrease the bendability by increase their tensile reduction of areaBendability also depends on the edge condition of the sheetImprove resistance to edge cracking by removing the cold-worked regionsCold rolling results in anisotropy by preferred orientation or mechanical fibering

44 Bending Sheets, Plates, and TubesSpringbackPlastic deformation is followed by elastic recovery when the load is removed, called springbackSpringback can be calculated by

45 Bending Sheets, Plates, and TubesCompensation for SpringbackSpringback is compensated for by overbending the partOne method is stretch bending where the part is subjected to tension while being bentBending ForceExcluding friction, the maximum bending force, P, isFor a V-die, it is modified to

46 Miscellaneous Bending OperationsExamples of various bending operationsRoll BendingPlates are bent using a set of rolls.Curvatures can be obtained by adjusting the distance between the three rolls

47 V-Bending and Edge BendingLow productionPerformed on a press brakeV-dies are simple and inexpensiveEdge-Bending:High productionPressure pad requiredDies are more complicated and costly

48 Miscellaneous Bending and Related OperationsBeadingPeriphery of the sheet metal is bent into the cavity of a dieThe bead imparts stiffness to the part by increasing the moment of inertia of that section

49 Miscellaneous Bending and Related OperationsFlangingIn shrink flanging, the flange is subjected to compressive hoop stresses and cause the flange periphery to wrinkle

51 Press Brakes – Bending EquipmentSheet metal or plate can be bent easily with simple fixtures using a presscomplex bendswith long narrow bed and short adjustable strokesmetal bent between interchangeable dies

52 Roll Forming Also called contour-roll forming or cold-roll formingUsed for forming continuous lengths of sheet metal and for large production runsDimensional tolerances, springback, tearing and buckling of the strip have to be considered

53 Bending of Tube StockStretch bending of tube: (1) start of process and (2) during bending

54 Tube BendingLarge dia. thin wall, with small bend radius tend to cause wrinkle at inner side of the tubeOldest method of bending a tube is to first pack its inside with loose particles and then bend it into a suitable fixtureThick tube can be formed to a large bend radius without the use of fillers or plugs

55 Tube Formingtubular part placed in a split-female die and expanded with a polyurethane or rubber plugpunch retracted; plug returns to its original shape and removed by knockout rodfinished part removed by opening the split die (water pitcher)production of fitting for plumbing

57 Stretch Forming Sheet metal is gripped by two sets of jawsThe jaws stretch the metal sheet and wrap it around a form block (die)Most of deformation is induced by tensile stretching, and forces on the form block are far lessVery little springback resultsForm block often made of wood or low-melting metalProduce large parts in low or limited quantity

58 Stretch FormingSheet metal is clamped along its edges and then stretched over a male dieDie moves upward, downward, or sidewaysUsed to make aircraft wing-skin panels, fuselages, and boat hulls

61 Deep DrawingMaterial beneath punch remains unaffected and becomes cup bottomCup wall is formed by pulling the remainder of disk inward the radius of die- Hoop stress tends to cause buckling or wrinkling - Pressure ring is used to suppress wrinkle

62 Deep DrawingWrinkling can be reduced if a blankholder is loaded by maximum punch forceThe force increases with increasing blank diameter, thickness, strength and the ratio

63 RedrawingCup becomes longer as it is redrawn to smaller diameters since volume of the metal is constantIf the shape change is too severe, more than one drawing step is required.The second drawing step, and any further drawing step is referred as redrawing

64 IroningIf the clearance between the punch and the die is large, the drawn cup will have thicker wallsThickness of the cup wall can be controlled by ironing, where drawn cup is pushed through one or more ironing rings

65 Deep Drawing: Deep DrawabilityEaringIn deep drawing, the edges of cups may become wavy and the phenomenon is called earingEaring is caused by the planar anisotropyPlanar anisotropy of the sheet is indicated by

66 Deep-drawing PracticeEaringToo high a blankholder force increases the punch force and causes the cup wall to tearDraw beads are needed to control the flow of the blank into the die cavity and reduce the blankholder forces

67 Deep-drawing PracticeCASE STUDY 16.1Manufacturing of Food and Beverage CansAluminum beverage cans has excellent surface finishDetail of the can lid is shown

68 Rubber FormingDies are made of solid materials, such as steels and carbidesThe dies in rubber forming is made of a flexible material (polyurethane membrane)In the bending and embossing of sheet metal, the female die is replaced with a rubber pad

69 HydroformingIn the hydroform, or fluid-forming process, the pressure over the rubber membrane is controlled throughout the forming cycleControl of frictional conditions in rubber forming is a factor in making parts successfully

70 Rubber Forming and HydroformingIn tube hydroforming metal tubing is formed in a die and pressurized internally by a fluid, usually waterRubber-forming and hydroforming processes have the advantages of:Capability to form complex shapesFlexibility and ease of operationLow tooling cost

72 SpinningSpinning is a process that involves the forming of axisymmetric parts over a mandrelA circular blank of flat sheet metal is held against a mandrel (form block of desired shape) and rotated while a rigid tool deforms and shapes the material over the mandrel; Disk of sheet metal progressively shaped by localized pressure with small rollerSuitable for conical and curvilinear shapes

73 Shear Spinning (Forming)A simplified version of the spinning process in which each element of the blank maintains its distance from the axis of rotation.Metal flow is entirely in shear and no radial stretch has to take place to compensate for the circumferencial shrinkage

74 Shear SpinningAlso known as power spinning, flow turning, hydrospinning, and spin forgingUse to produce an axisymmetric conical or curvilinear shape while reducing the sheet’s thickness and maintaining its maximum (blank) diameter

75 Tube Spinning Tube SpinningThe thickness of hollow, cylindrical blanks is reduced by spinning them on a solid, round mandrel using rollersCan be carried out externally or internallyVarious external and internal profiles can be produced from cylindrical blanks with constant wall thickness

76 Spinning Incremental FormingSimplest version is incremental stretch expandingA rotating blank is deformed by a steel rod with a smooth hemispherical tip to produce axisymmetric partsCNC incremental forming uses a CNC machine tool to follow contours at different depths across the sheet-metal surfaceAdvantages are low tooling costs and high flexibility in the product shapes

77 Explosive FormingUse of explosive charge to form sheet (or plate) metal into a die cavityExplosive charge causes a shock wave whose energy is transmitted to force part into cavityApplications: large parts, typical of aerospace industry(1) Setup, (2) explosive is detonated, and(3) shock wave forms part and plume escapes water surface

78 Explosive Forming The peak pressure, p, is given byThe mechanical properties of parts similar to those made by conventional forming methodsThe dies may be made of aluminum alloys, steel, ductile iron or zinc alloysp = pressure, psiK = constant that depends on the type of explosive

79 Explosively Formed PartExplosive used as a source of energyRapid conversion of explosive charge into gas generates a shock wavePressure of shock wave is sufficient to form sheet metalno limit to the size of the workpiece (suitable for low quantity of large parts, ie aerospace application)

80 Electromagnetic Formingcoil current rapidly discharged from capacitor bankeddy current generated in the tube (workpiece)repelling force between the coil and the tubeforces generated collapse the tubeHigher the electrical conductivity of the workpiece, the higher the magnetic forces

81 Peen FormingUsed to produce curvatures on thin sheet metals by shot peening one surface of the sheetSurface of the sheet is subjected to compressive stressesThe process also induces compressive surface residual stresses, which improve the fatigue strength of the sheet

82 Laser FormingInvolves the application of laser beams as a heat source in specific regions of the sheet metalProcess produce thermal stresses, which can cause localized plastic deformation of the sheetIn laser-assisted forming, the laser acts as a localized heat source, thus reducing the strength of the sheet metal at specific locationsImprove formability and increasing process flexibility

83 Superplastic FormingThe behavior of superplastic are where tensile elongations were obtained within certain temperature rangesSuperplastic alloys can be formed into complex shapes by superplastic formingHave high ductility but low strengthAdvantages:Complex shapes can be formedWeight and material savingsLittle residual stressesTooling costs are lower

86 Manufacturing of Metal Honeycomb StructuresA honeycomb structure has light weight and high resistance to bending forces, used for aircraft and aerospace components2 methods of manufacturing honeycomb materials:Expansion processCorrugation process

87 Manufacturing of Metal Honeycomb StructuresA honeycomb structure consists of a core of honeycomb bonded to two thin outer skinsHas a high stiffness-to-weight ratio and is used in packaging for shipping consumer and industrial goods

88 Design Considerations in Sheet-metal FormingBlank DesignPoorly designed parts will not nest properlyBlanks should be designed to reduce scrap to a minimum

89 Equipment for Sheet-metal FormingPress selection for sheet-metal forming operations depends on:Type of forming operationSize and shape of workpiecesNumber of slidesMaximum force requiredType of mechanical, hydraulic, and computer controlsFeatures for changing diesSafety features

91 Economics of Sheet-forming OperationsSheet-forming operations are versatile and can produce the same partThe costs involved depend on die and equipment costs and laborFor small and simple sheet-metal parts, die costs and lead times to make the dies are lowDeep drawing requires expensive dies and toolingEquipment costs depend on the complexity of the forming operation